The field of this disclosure relates to processes for suppressing minority carrier lifetime degradation in silicon wafers for use. In particular, the field of the disclosure relates to efficient and cost effective processes for reducing the concentration of minority carrier lifetime degradation defects itself.
Solar cells and, particularly, cells manufactured on boron containing p-type single crystal silicon wafers grown by the Czochralski method and also, typically to a lesser extent, multicrystalline wafers grown by casting degrade in performance when exposed to light or when minority carriers are injected into the cell in the dark. This performance degradation continues until a stable efficiency well below the initial efficiency is achieved. The efficiency loss in CZ wafer-based cells may be up to about 10% or more. Such loss of efficiency in Czochralski solar cells limits the potential for high efficiency silicon cells and the use of single crystal silicon in the industry since Czochralski cells are typically more expensive to produce than multicrystalline silicon-based cells.
It has been found that this degradation may be at least temporarily reversed by annealing the wafer for a few minutes at a low temperature such as an anneal at about 200° C.; however, the increase in efficiency achieved by the low temperature anneal is lost upon subsequent illumination. It has been reported that the degradation may be permanently reversed by injecting excess electrons into the wafer (e.g., by illuminating the wafer) during a low temperature anneal in the range between 50° C. and about 230° C.); however, for permanent recovery to occur under these conditions the low temperature anneal must occur for relatively long periods of time (e.g., several tens of hours).
A continuing need exists for commercially practical methods for permanently suppressing the degradation defect that is related to light induced degradation in solar cells.